There has been a biaxial driving machine in which a moving table is supported by two ball screws disposed in parallel, and is driven by synchronously controlling two servomotors that are connected respectively to the two ball screws. The moving table in the biaxial driving machine installs, in general, an upper structure that is movable, and the center of gravity of the moving table varies according to the position of the upper structure, the load supported by the upper structure, and the like.
Moreover, a position control device for a motor has been disclosed in Patent Document 1 (Japanese Patent Laid-Open No. 30578/1994) as related to position control devices for motors (direct-current motors, induction motors, synchronous motors, etc.) that drive load devices such as a table in a machining apparatus and arms of an electric industrial robot.
Patent Document 1 aims to realize a position control device for motors, which has a high-speed response, prevents throb of machinery, and has a constant response frequency at all times even if the inertia moment of the load device varies. In the position control device for motors in Patent Document 1 (line 23 through line 36 in the right column in page 5), a first velocity signal is acquired by means of a first position control circuit, and a second velocity signal is acquired by means of a second position control circuit for controlling a mechanical simulation circuit. In addition, the first and second velocity signals are added to acquire a third velocity signal, and a first torque signal is acquired by means of a first velocity control circuit, to which the third velocity signal has been inputted. Likewise as above, second and third torque signals are acquired by means of second and third velocity control circuits, and the torque generated in the motor is controlled so as to follow the final torque signal calculated by adding the first through the third torque signals.
The position control device for motors in Patent Document 1 includes the second position control circuit, the second velocity control circuit, and the mechanical simulation circuit, whereby there is a benefit in that the response of the positional control with respect to the changing of a rotation angle command signal can be enhanced. In addition, by including the third velocity control circuit, there is a benefit in that the response of the positional control with respect to the changing of the load torque can be also enhanced.
Moreover, as related to control systems for synchronously positioning legs on both ends of a machining apparatus having a movable structure of a gate form or the like, a synchronous position control device has been disclosed in Patent Document 2 (Japanese Patent Laid-Open No. 226206/1987), which aims to achieve a synchronous position control system that does not generate asymmetry due to disturbance.
In Patent Document 2 (line 3 through line 10 in the lower left column in page 2), in a system in which on both sides of the movable structure, position control systems that move along both the sides according to position command values are provided, and in which the position control systems synchronously position both ends of the movable structure, a problem of asymmetry due to disturbance has been resolved by calculating the difference between movement positions of both the ends of the movable structure, calculating compensation values by applying compensation elements on the difference, and feeding back the compensation values to the respective position control systems.
When the position control device for motors in Patent Document 1 described above is applied to control of a biaxial driving machine, if load inertias of two axes are balanced, the first axis and the second axis identically move, whereby appropriate driving can be performed. However, when the inertias of the two axes are unbalanced, because movements of the first axis and the second axis do not coincide, a deviation (between-axes positional deviation) arises between the actual position of the first axis and the actual position of the second axis. There has been a problem in that the between-axes positional deviation degrades positioning accuracy of the moving table and generates stress on the machine, which shortens the machine life, and may cause damage on the machine at worst.
Therefore, there has been a problem in that the position control device for motors in Patent document 1 described above cannot be applied to the control of a biaxial driving machine in which large load unbalance is present.
Meanwhile, the synchronous position control system in Patent Document 2 described above feeds back the difference between the positions in the two position control systems (between-axes positional deviation) to the two position command values to resolve asymmetry. When the control system is applied to the control of a biaxial driving machine, a certain level of the between-axes positional deviation can be suppressed; however, in order to suppress a large between-axes positional deviation, a response of a position command correction unit (coefficient H1 here) must be set to an extremely large value. When the response of the position command correction unit is set to an extremely large value, although a high-rigidity machine has no problem, a low-rigidity machine has a problem in that the system sometimes gets unstable, because oscillation of the machine is caused, for example.
Moreover, setting the gain to a high value sometimes encourages a conflict between axes, which is generated when position detection errors or the like are present. There has been a problem in that, when a conflict between axes arises, the motor generates heat due to the pulling torque between axes, which prevents high-speed and highly accurate control, and the machine might be damaged due to the pull between axes at worst
Therefore, there has been a problem in that the synchronous position control device in Patent Document 2 described above cannot be applied to a low-rigidity machine or a machine in which position detection errors are present.
The present invention has been made to resolve the problems described above, and aims to realize a servo controller that is applicable to a low-rigidity machine or a biaxial driving machine in which position detection errors are present.